MacBook Pro :: No Boot When HDD Inside Machine, Just Fine In External Enclosure?

I have a mid-2009 13" MacBook Pro that's having a strange problem. It crashed hard last night while I was typing something- just froze solid, couldn't force quit, then the mouse pointer stopped. Shut it down with the power button, started it again, got the "not permitted" symbol (circle with a line through it). Restarted with my Tiger emergency USB boot drive and the hard drive didn't show up in Disk Utility. Whoa! Restarted with a Cmd - Opt - P - R, no change. 

I pulled the hard drive and put it in an external enclosure that I have. It mounted just fine on my iMac so I backed up all of my data, then plugged the drive in to the MacBook to see what would happen-and behold the MacBook booted just fine with the drive attached in the external enclosure! It worked perfectly! At this point, I had no drive installed in the MacBook, the MacBook's hard drive was attached via an external USB enclosure, and I booted the machine via the Lion USB rescue utility. I ran Disk Utility thinking that it would have all sorts of errors to report, but the partition showed no problems. I repaired permissions for good measures, then put the drive back in to the MacBook. Again, I got the "not permitted" symbol. 

I tried booting from the USB rescue drive- when I went to Disk Utility the drive showed up but the system hanged when I selected it. I restarted with the Snow Leopard DVD inserted but it froze before loading the setup process (I assume when it was trying to mount a file system from the HD). 

I run Windows 7 in a Boot Camp config on this machine as well, so just for fun I tried to start from the Windows partition. To my surprise, it booted just fine from Windows. On top of that, once in Windows I had no trouble accessing the Mac partition. 

I decided at that point that maybe something had gone awry that I couldn't see and that I should repartition the drive. I put it back in the external enclosure, booted the MacBook from the Snow Leopard DVD, used Disk Utility to repartition the drive, put it BACK in the MacBook and tried to boot from the Snow Leopard disc again- and it crashed in the same place. I rebooted from the Lion rescue drive and tried to use Disk Utility to check the drive- DU crashed when I selected the hard drive. 

As a last-ditch effort, I swapped out the RAM and tried again. No go. At this point, I'm really hoping it's a problem with the hard drive, but seeing as though it works great from an external enclosure, I don't know what to think. I don't have any other 2.5" SATA drives to test in the machine so I have to assume that other than a failed SATA controller, firmware, or other logic board problem, it must be the drive, right? 

Info:
MacBook Pro (13-inch Mid 2009), Mac OS X (10.7.3), 4 GB RAM, 160 GB Hitachi HD